When a subject to be photographed is bright, the opening process of the cmaera shutter until the opening blade is fully released will vary together with an elapse of time. The aperture diameter and the time will be determined according to the brightness of the subject. When the subject to be photographed is dark, only the time will vary after the opening blade is fully opened. Thus, exposure is determined automatically according to the brightness of the subject. The .gamma. characteristic of the photoconductive element of the above mentioned programming shutter is not linear as will be seen from curves I and II of FIG. 3. When the subject to be photographed is bright, that is within the range in which the opening blade is not fully opened, and an aperture diameter and a time are determined simultaneously, the value of .gamma. must be smaller than 1 while within the range when the subject to be photographed is dark and only the time varies, after the opening balde is fully opened, the value of .gamma. must be approximately 1.
The introduction of an exposure multiple in a programming shutter requiring the above mentioned characteristics of a photoconductive element cannot be achieved by the conventional methods of changing-over the time-limiting capacitors of the shutter circuit or changing the switching levels in switching circuits. The relationship between brightness and the resistance value of a photoconductive element for different film sensitivities is illustrated in FIG. 3. Assuming that curve I represents the relationship between brightness and the resistance value of the photoconductive element for film having a ASA rating of 100, the relationship in the case of ASA 400 is represented by curve II. The curves show the resistance values of the photoconductive elements at points where the EV value in both cases is identical. There is the following relationship between the curves of ASA 100 and ASA 400: EQU r evn = R' EV(n - 2)
where,
______________________________________ R: Resistance value of ASA 100 R': Resistance value of ASA 400 ______________________________________
When it is attempted to introduce a film sensitivity in the manner mentioned above, by changing-over time-limiting capacitors or changing switching levels, the above formula will be realized within the range of .gamma.=1 but not within the range of .gamma.&lt;1. For instance, assuming that a 1/4 capacity of ASA 100 is used, when the capacity of a time-limiting capacitor is ASA 400, the time determined in the RC circuit will be 1 : 4. If taking this as the resistance value of the photoconductive element varying apparently, EQU R 400 = 1/4 R 100
where,
______________________________________ R 400: resistance for ASA 400 R 100: resistance for ASA 100 ______________________________________
A resistance for ASA 400 or R 400 is as shown by the dotted line curve III in FIG. 3, always in relation of 1/4 to the resistance R = 100 of ASA 100 in Ev value. Accordingly, the formula may be brought into realization within the range of .gamma.=1 but within the range of .gamma.&lt;1 as explained above and as is clear from FIG. 3, the error will become larger.